Method of and apparatus for burning fuel



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M. A. MAYERS METHOD OF AND APPARATUS FOR BURNING FUEL Filed Sept. 2s, 1939 March 17, 1942.Y

H rT-OR/VEXIS March 17, 1942. M. A. MAYERS METHOD oF lAND APPARATUS Fon BURNING FUEL Filed Sept. 28, 1939 4vSheets-Sheet 2 i INVENToR f Martin Mayers March 17, 1942. M. A. MAYERS 2,276,327

\ METHOD' OF AND APPARATUS FOR yBURNING FUEL FiledfSept. 28, 1939 4 Sheets-Shet 4 N VA TTO/ENEYS March 17, 1942. M. A. MAYERS 2,276,327 f METHOD OF AND APPARATUS FOR BURNING FUEL Filecrseptl 28, 1959 4 sheets-sheetz.

,CQK//v G ZONE Bun/vm@ LANE 9 I' o 0O o C OO 90 @Q70 Q il@ C9 o@ h@ O. f? A l I e i ss i s i i l INVVENTOR' Marin-Mayera mwm Patented Mar. 17, 1942 METHOD OF AND APPARATUS FOR BURNING FUEL Martin A. Mayers, Pittsburgh, Pa., assignor to Carnegie Institute oi Technology ot Pittsburgh, Pittsburgh,`Pa., a corporation of Pennsylvania Application September 28, 1939, Serial No. 296,934

(ci. 11o-'44) 17 Claims.

My invention relates in general to a method of and apparatus for burning fuel and is particularly applicable to underfeed stokers and especially to large, multiple retort underfeed stokers such as are used in connection with steam boilers which use coal as the fuel.

The grate in a multiple retort underfeed stoker is made up of a number of fuel retorts and tuyre stacks arranged in parallel lines side by side and extending from the front of the grate to the rear thereof. My invention consists in the arrangement of apparatus and a method of burning coal in such a manner that th'e rate of burning of the fuel per unit of grate area is substantially greater than that heretofore obtained. I have discovered how to determine the correct width of the tuyre stack and of the retort for any desired combustion rate of a desired fuel. Stokers constructed following the'teachings of my invention have retort sections, the width of wh'ich is much less than that of existing stokers and is of much greater eiiiciency and capacity.

In the accompanying drawings I have shown for the purposes of illustration only the preferred Figure 3 is a horizontal section partly diagrammatic showing the grate area;

Figure 4 is a view in perspective of a tuyre;

Figure 5 is a vertical section to an enlarged scale along the line V-V of Figure. 1 showing two retorts and associated tuyres;

Figure 6 is a view along the line VI-VI of Figure 1 showing a detail of the apparatus; and

Figure 7 is a view largely diagrammatical showing apartial section through a retort and tuyre and a part of the overlying fuel bed of a conventional Stoker.

It has heretofore been thought by those skilled in the art that fuel beds of underfeed stokers were essentially uniform all the way across the stoker and divided into substantially horizontal strata and that the green coal which is pushed up into the coking zone is gradually coked by the heat from the fire above. I have discovered that such is not the case, but, on the contrary, (1) that the fuel bed is not continuous but is divided into subthe retort is not heated, excepting in a small restantially vertical strata which extend longitudinally of the grate and consist of a multiplicity of parallel channels 4extending along the grate immediately above the tuyre stacks in which active burning ofthe fuel occurs, the channels being separated by elongated heaps of coal and coke in the retorts in which almost no burning occurs;

(2) that the coal is not heated appreciably from and, lastly, (5) that the level of the fuel bed is A seldom more than one to two feet deep above the level of the tuyres.

vThese discoveries can best be visualized by reference to Figure 7, which represents a cross section of a portion of a conventional stoker taken through a tuyre section T and a stoker retort S. On this diagram, which it is understood represents only a portion of the cross section and is repeated as many times as there are tuyre stacks in the stoker, are shown contour lines representing regions of equal temperature. v These contour lines are the traces on a vertical plane of surfaces extending parallel to the axes of the retort and tuyre stack and vbounding the regions of equal temperature throughout the length of the stoker. It will be observed that th'e coalfin the center of gion atthe top of the retort. Passing from the `center line of the retort toward the center line of the tuyre stack the temperature increases 4through the coking range of the fuel and finally reaches very high' values in the channel above the center line of the tuyre stack through which the air for combustion passes, maintaining very rapid burning on the Walls of this channel. The

region within the heavy line sh'own is a compact mass consisting of green coal G within the dash line between the contours at 95 and at 1000 F.

and of massive, nearly monolithic walls of coke C between the dash line and the heavy outline. The dash line represents the approximate locus of the coking region. The channel'D through which air flows up through the bed is nearly empty and most of the combustion takes place on the walls of this channel.

By means of these discoveries, I have been able to yanalyze the conditions underlying the flow of heat into the coal which is being prepared for combustion in the retorts ofthe underieed stoker, and I can show thatthis heat flow must obey laws similar to those governing the flow of heat into the charge of a slot-type byproduct coke oven. For, as shown in Figure 7,

the steepest temperature gradients and hence, the mostl rapid heat ow, appear in a horizontal direction across the retort so that the principal ow of heat resulting in coking green coal takes place from the burning lanes on either side of the retort toward the center of the retort. This in a byproduct coke oven, as shown by the researches of D. W. Wilson (Proc. Am. Gas. Assoc.,

vol. 1923, pp. 952-64), W. P. Ryan- (Ibid, vol.'

19,25, pp. 861-78) and others except that in the byproduct coke oven the source of heat is the confining brick wall of the oven which is heated by the combustion of gas in ilues passing through it. This difference in structure can obviously have no effect on the laws governing heat penetration within the mass of coal and coke.

Now it has been found empirically by Dr. H. H. Lowry (unpublished data communicated privately) from the correlation of many data published by the U. S. Bureau of Mines in Monograph No. 5 (1934) by A. C. Fieldner and J. D. Davis, and checked against other published data (e. g., Coke for Blast Furnaces by R. A. Mott and R. V. Wheeler (1930) Table 88, page 215) that the time required for coking a charge of coal in a byproduct coke oven, ceteris paribus, varies as the 1.6 power'of the oven width or diameter. This is an empirical fact whose explanation is not, yet understood, but which we are now seeking. Making use of this fact, and the analogy shown above to exist between heat flow in an oven and in the retort'I of an underfeed stoker we may write that the time required to coke the coal in a retort of width w is given by mwLe where m is a parameter depending on the coal characteristics, the temperature of the source, and other such quantities and need not be more exactly specified since it will be eliminated during this derivation. Now, the amount of coke formed in such a retort per unit length is proportional to the width of the retort and may be written as nw, where n. depends on the coal characteristics and the height of the retort. Hence, it is evident that the average rate of coking per unit length of retort is given by the quotient of these expressions or "m05 00.6 where B is a constant depending on the character of the coal (fixed carbon, ash and moisture content), the height of the retort, the temperature in the burning lanev and perhaps upon other factors. The constant B may be defined as the rate of coking per foot of retort length for a retort one foot wide.

In view of the discussion of the operation of such underfeed 'stokers which will be given later, it is evident that on the average each tuyre stack must burnl coke at the rate at which it is prepared and delivered to it by the retorts on either side of it. Now the rate of burning on the tuyre stacks referred to unit area of the stack may be measured for any particular coal and any particular installation, as hereinafter pointed out or it may be calculated from the formula for U* given below, which was derived by me in a paper presented before the American Society of Mechanical Engineers (Trans. A. S. M. E., vol. 59, pp. 279-288 (1937); see particularly page 281, equation 15).

where Hence, the rate of burning on a tuyre stack whose width is d, will be, per unit', length of tuyre Ud. Since this must, as shown above, be equal to the rate at which coke is delivered to the tuyre stack, we may, for a correctly pro portioned stoker.. set

an: U*d

which results in the first proportioning formula dw-=g (1) The quantity B depends on the properties of coal as a heat conductor, the temperature obtaining at the face of the heap of coke, and the height of this facelthrough which heat is conducted into the -coking coal.' I have determined by experiments suitable values of this quantity B for typical high and low volatile bituminous coals coking in a region that extends one foot above the level of the tuyres. For these conditions, a suitable average value of B is 90, when U* is expressed ln pounds of coal per hour per square foot ofair admission surface. For higher or lower coking faces, the quantity B must be increased or decreased respectively in proportion to the depth. Because of' variations in the properties of different coals, B is not a true constant, but may be expected to vary over a range of 25% greater or less than the value v stated, say, from about 65 to about 120 per foot height above the level of the tuyres.

'I'he eil'ect of coal characteristics on the value of B Acan be estimated as follows: B is inversely proportional to the sum of ash plus fixed carbon in the coal and is decreased approximately 3% for each per centof moisture content. It depends *upon the temperature in the burning lane of the Stoker in which the value of B is being determined or applied. It increases with r the temperature approximately as indicated by the equation 10g B=log 3,-@

Method 1 Measurement is made of the rate of the transverse flow of coal in an underfeed Stoker of any suitable type by the method described in the article by M, A. Mayers, W. H. Dargan, J. Gershvelocity to fuel flow in berg, B. C. Dalway, M. J. Williams and E. R. Kaiser, the article being entitled 'I'he Fuel -Bed Tests at Hell Gate Generating Station, and read in 1938 before the American Society of Mechanical Engineers (to be published in their Transactions shortly). In these tests, motion pictures of the bed were taken from the rear wall of the furnace from which the transverse velocity of the fuel flow could be estimated. The tests were made while the furnace was in a steady condition and the transverse velocity offuel across retorts 11" in width, in which the coke mass was estimated to be 16" wide was 0.7 foot per hour. Since under the conditions of the test the fuel bed was in a relatively steady state, the rate of transverse flow of the fuel was equal tov the rate of coking, so that these data can be inserted in the expression for the rate of coking l v 0.6 with the factors necessary to change from linear pounds per foot of retort length giving the following: 2 (factor) 03 (transverse velocity of fuel) x1 (height in feet of coke above tuyres) x45 (pounds per cubic foot of coal) Method 2 A second method of determining B for any particular coal is as follows: The coal is burned in an underfeed stoker of An examination of this formula will show that the average rate of burning increases more and more as w is decreased by comparison with d. Thus it will be seen that it is advantageous to make w as small as possible in view of the pracseen that U* any suitable type and the burning rate of the coal? i in the stoker is determined. This burning rate can be determined by a weighed coal test, or when the boiler emciency is known, can be calculated from the rate of steam production.

Assume that the stoker used in determining the burning rate of the coal had the following dimen-sions and that the rate of burning coal was 12,200 lbs/hr.:

1. Total stoker area=382 square feet 2. Total air admission area=182 square feet- 3. Width of tuyre stack (d) =101/2 in.

4. Width of coking mass (w)=16 in. (estimated) The average rate of burning ofthe coal in pounds per hour per square foot of air admission area equals 12,200/182=67.1. This value of 67.1

is a measured value of U. Therefore,

, U*d=67.1 X%5=58.7 lbs/ft. length of tuyre Referring now to the first proportioning formula,

and transposing in order to solve for B, we have vthe formula B= U*d w These discoveries lead to another formula tical requirements set by the necessity of feeding coal.

By the use of these formulas it can be shown that the maximum coal burning rates that are possible with conventional designs of stokers have been reached. In conventional stokers, as marketed by the principal manufacturers of such equipment, d is equal to about ll inches and w is usually about 10 inches. There are slight variations in the designs of diiferent manufacturers, i

but these variations do not exceed an inch or two greater or less than4 the dimensions given. Using these dimensions in Formula 1, it will be has a value oi 110, which, using Formula 2 corresponds to a value of U of about 60 pounds/square foot/hour. The maximum coal burning rates that have been attained on multiple retort underfeed stokers are reported in the paper Ten Yearsof Stoker Development at Hudson Avenue, John M. Driscoll, W. H. Sperr, Trans. A. S. M. E., 57, 49-58 (1935) as about 75l pounds/square foot/hour. This figure is referred to the projected area of the furnace and includes the area occupied by the ash pit. Correcting for the width of the ash pit and for the slope of the stoker furnace, this corresponds to the same value combustion rates, but does so at the risk of ineillcient combustion and the production of smoke. If the average ligure of '75 pounds/square foot/hour given above were corrected for the high combustion rate developed onl the extension grate, the quantity U=60 pounds/square foot/hour for the underfeed section would not be materially exceeded.

By the use of the proportioning Formulas 1 and 2 given above, it is possible to design stokers for values of U* much greater than the value of which is found to be the limit for conventional stokers. Thus, my discoveries and the use of the proportioning formulas permit the exten- Ision of U* to values greater than 150. There is, however, an upper limit to attainable values of total area of the U*, which is set by the necessity of operating at air flow rates below those which will disrupt the fuel bed. This upper limit becomes effective at values of U* in the neighborhood of 250 to 300, depending on' the size of the particles of coked fuel delivered by the retorts. In order to show the .advantages that may be gained by the use of these formulas, a single example will sumce. Setting w=3 inches and d=10inches in Formula 1 and solving for U* we obtain U*=250. Putting this value and the values of w and d in Formula 2, we obtain U=200. This is a 300% increase over the maximum combustion rate attainable von the underfeed section of multiple retort stokers as they are now built. It is seen that the width of the retort which-I propose is only a fraction of the width which has heretofore been used and considered necessary.

The dimensions of the stoker can be determined as follows: Using the Formulas 1 and 2, calculate U for the fuel to be used and the conditions of firing, that is, the air pressure available, size consist of fuel, depth of fuel bed; divide the load to be carried in pounds of coal per hour by `U and this will give the required stoker area. It is desirable to make the grate as nearly square in cross section as possible, although this proportioning is not essential.

It is understood that this example is merely typical of the results that may be attained by the use of the proportioning formulas given above.

.In order to understand the functional meaning of the improvements about to be described, it will be necessary to bear in mind the operation of existing types of underfeed stokers. The explanation will be confined to a description of the process occurring between the center line of a retort and that of the next` adjacent tuyre stack in any of the common types .of multiple retort underfeed stokers, from which the entire stoker can be built up by simple multiplication of units. In such a stoker, raw coal is fed into the retort and is pushed along the retort until it rises above the level of the tuyre stack which slopes down to meet the level of the bottom of the retort. From the burning lane over the middle of the tuyre stack (which, however, may be displaced from 'side to side across the width of the tuyre stack by uncontrolled variations of the properties of the fuel or of the feeding or burning action of the stoker) which acts as a source of heat, heat is conducted into the coal in the retort and a coking zone travels across the retort from the burning lane, reaching the center of the retort by the time the coal has reached the end of the retort immediately adjacent to the ash pit or extension grate, if any, when the stoker is properly operated. On the cold side of the coking zone, i. e., toward the center 4of the retort, the coal remains substantially unheated, even when it has passed almost all the way down to the ash pit extension grate. On the hot side of the coking zone, the semi-coke continues to be heated until, by the vformation of shrinkage cracks and the agitating action of the secondary rams, portions of the coke wall formed by the passage of the coking zone break off and fall down into the burning lane producing a more or less continuous porous fuel bed through which the air stream introduced through the tuyres may pass. Since the fuel falling into the burning lane has already been heated to ignition temperature, the air stream causes it to burn, .releasing heat. In this manner, the self-supporting fuel bed is set up and it continues to burn so long -as fuel is supplied to the retorts and air is supplied to the tuyres.` I have found that the limiting factors in this operation are (1) the slow rate of heat penetration into the raw coal supplied to the retorts; (2) channeling of the fuel bed over the tuyre stack causing the appearance 4of marked burning lanes which may contain very little fuel; and (3)y general instability of the fuel bed causing large areas of the bed over the tuyre stack to be blown clear of fuel thus allowing large amounts of air to enter the furnace without being made to support combustion and thus be heated, and exposing the grates to dangerous heating by radiation from the surrounding active fuel bed.

The most serious of these shortcomings is the first which in itself limits the attainable rating on most existing stokers and is the principal reason for the existence of the other two which occur principally because of the failure of a sufficient supply of prepared coke for the burning lanes. It may also be noted that operation according to this scheme involving the overfeed of fuel to the burning lanes permits the attainment of extremely high temperatures, between 2800 and 3100 F. in the burning lanes. At such temperatures any small particles of ash released in the burning lane will be immediately ducing slagging of the tubes.

The improvements suggested here, and therein A at all of these limlies my invention, are aimed iting factors as follows:

1. The retort is to be made as narrow 'as conveniently possible considering the size of coal which it is desired to burn. This step will, as explained in more detail above, increase considerably the rate of heat penetration into the cold fuel. i".

2. Violent agitation is to be applied to the fuel in the region between the retort and the tuyre stack to break up the coke wall as rapidly as formed into small uniform pieces. In this way, the width of the coal mass is maintained substantially equal to the width of the retort. This step is aimed at the third limit mentioned above. It does, however, introduce a difiiculty due to the very rapid release of volatile substances from the coking coal which is, in turn, v

taken care of by improvement 4, infra.

3. The active grates or tuyre stacks are to be made just wide enough so that the coke formed at the rate of which the retort is capable can be burned with a deficiency of air when the airis introduced at a rate high enough to cause the bed to have the consistency termed by Hirst (Trans. Institute of Mining Engineers, 79, 463, ibid 85, 236) just moving.

Under these conditions the bed is sufficiently fluid so that it will flow to a uniform level. When this condition of just moving" is maintained by the use of an interrupted stream of air, the

uniform level'of the bed can be maintained with.v

arrasar bed. This dimculty is also met by improvement 4, infra.

d. A large proportion oi the air for combustion is to be applied over the nre, either in the form of jets under high pressure, or air introduced tangentially under pressure so as to cause a vortex just above the burning fuel. By a large proportion is meant from 25% to '10% of the total quantity of air supplied, depending on the proportion of volatilematter in the coal, the re activity of the coke formed from the coal, and the thicknes of fuel bed it is desired to carry.

5. The ash is to be removed from the bottom of the fuel bed at the foot ci the burning lane in much the same way as refuse is removed 'from a coal-cleaning table. ri'his is possible because oi the classincation by density that occurs in a iluid bed such as that in the just mcvmg condition.

it is apparent that the air iiow rate will'not .affect the penetration oi heat into the iuel in the retort because the heat hflow depends only on the thickness and cross sectionai area of the coal layer and on the temperature maintained at its face. This temperature does not change appreciably 'in the lower part or the bed adjacent to the coking zone since it is determined largely by the heat of reaction of the fuel and its direction of ow. The fuel iiow in this region will continue to be overfed as in existing stokers, so the temperature will rise to about 2700* F. Only beyond this region will the temperature, which in existing stokers reaches 3160, decrease because of the increased importance of the endothermic reduction reaction C+CO2- 2CO which absorbsabout 580i) B. t. u./pound of carbon. Since there Y will be no flow of air vthrough the retort, the increased air iiow through the stolzer will not aiiect the rate of heating oi the coal. Maintenance of the area necessary to permit the required amount of heat to flow will be automatic pension of the bed gases, due to their rise in temperature, may-effect this quantity, althoughit y seems more likely that its eiect will be solely on the-rate of air i'low associatedwith the pressure required to produce just moving conditions; rlhe latter quantity depends only on the size of the fuel particles. For closely sized beds. the just moving" condition is reached at higher gas velocities than with unsized beds, although the pressure applied is nearly'the same, but there are no definite figures available on the iiow velocity in unsized beds.

In theA drawings, there is shown a multiple retort underfeed Stoker suitable for use in connection with a steam boiler, the sidewalls of which are designated by the numeral 2, the front wall b'y the numeral t, and the vback wall by the numeral t. Thegrate t is made up oiv a number of units, each consisting of a tuyre t and a stoirer retort i. arranged side by side and extending from. l

the :iront wall s of the boiler to a point adjacent the rear wall The number and length oi theunits depend on the desired capacity oi the boiler. The length of the grate and the rate of fuel feed would be adjusted to insure that the fuel will be colsed by the time it reaches'the end of the unit. The depth of the fuel bed is such that it may extend to the rear wall i of the furnace and the ash alone lsvorced over the end of the tuyre stack into the ash pit. The grate 5 terminates below a stop 9 and short of a jog in the rear wall t, thus leaving a space It leading to the ash pit t. The stop d extends into contact with the bottom 25 of the fuel retort.

because if it tends to become too small, the action of the agitating mechanism will raise'the uncoked coal in the retort thus increasing the area exposed. This process of heating in underfeed stokers produces the ,characteristic that rendersthem independent of the restrictions on the rate of ignition present in pure underfeed burning. G. Eilers, in Transactions of the American Society of Mechanical Engineers, vol. 56, p. 321, 1934. The term pure underieed burning is here lused as burning in which the iiow of air and the flow of fuel through the fuel bed takes place in the same direction, or alternatively in which the ignition of fresh fuel proceeds in a direction opposite to the ow of air. Thus, the rates of ignition in existing stokers are above those that could occur in pure underieed burning, and by the improvements described herein they are increased still further.

The consistency oi the fuel bed required by improvement No. 3 is obtained at a forced draft pressure that depends only on the density of the bed and its depth. More specifically the forced draft pressure should approximately equal the product of the bulk 'density times the depth of the bed so that a pressure gradient is maintained through the bed equal to its bulk density. For a bed of coal, this pressure gradient (Hirst, pre-A viously cited) has been determined as 0.67" wa- See the article by P. Nicholls and M.

ter column per inch of bed thickness. Since the bed in this case will be coke of somewhat lower' The tuyre 6 consists of a plurality of tuyre sections ll, each arranged end to end from the front of the grate to the back of the grate. One

yand has a tongue i2 depending from thesloping bottom of the section. The tongue is shorter than the width of the tuyre and both ends of the tongue terminate a short distance from the sides of the section. A number of suitable holes i3 are provided extending from the bottom of the section to the top. The section illustrated shows four holes which incline toward one another upwardly and terminate on opposite sides of the tongue l2. The sections ll are laid end to end and the sloping bottom portions rest on lugs it, which are secured to the partition plates i5 which separate the tuyres d from the retorts l. The plates extend from a position adjacent the top of the tuyre downwardly past the bottoms of the secondary rams it.

The partitions l5 are supported by and pivoted to the' upper ends of links ll. The lower ends of the links are pivoted on supports in the boiler adjacent the front and rear vof the grate 5. The

links are hingedly` secured both to the partition platesl and to the boiler supports so as to allow a reciprocating motion fof the partition plates lengthwise of the grate. The links il are moved back and forth vby the rod it, which has a quick return motion connection i9 with the shaft 263. The quick return motion connection i9 comprises a cam 2i having a projecting tongue 2t and a cutaway portion which allows the rod it to be forced rapidly backward by the .spring 23 after the cornpletion of the forward stroke. The cam 2i is secured to the shaft" t@ and is rotated by'it. The rotation of the shaft 2t causes the partition plate i5 and the tuyres supported thereon ,to be reciprocated relatively slowly in a direction toward shown).

the rear of the grate and relatively quickly toward the front of the grate on the return movement.

The stoker retorts 1 lie between the tuyres 6 and each is composed of a number of secondary rams I6, the top surfaces of which are stepped or saw-toothed, as illustrated in Figure 2. A portion of each secondary ram overlaps the secondary ram immediately in the rear of it and the lower front part of the secondary rams rest on a pilot bar 24, which is supported at its ends in the furnace structure and slopes upwardly from the front of the grate to the rear. The secondary rams are thus prevented from tipping and are compelled to move in a generally straight line which is upwardly inclined. The uppersurfaces of the secondary rams slope upwardly from the front of the grate to the rear of the grate and at the rear of the grate the top of the adjacent secondary ram is substantially level with the top of the tuyre section and overlies a plate 25 which `.extends between the partition plate I forming the side of each tuyre sectionand closing off the end thereof. 1 The secondary rams I6 are each driven by lever 26 associated with each section, which lever, in turn, is driven by an eccentric connection 21 which is driven by a shaft 28 geared to shaft 23. The shaft 29 is gear connected with the shaft 20. It will be observed by inspection of Figures 1 and 2 that the secondary rams I6 are so arranged that they are moved in a step-like motion relative to the ash bed along the grate can be regulated as operating conditions may require.

At each comer of the furnace Walls, there are provided openings or ports 42 through which air for combustion is supplied from the lbustle pipe '43, which extends around the sides and back wall of the furnace and which contains air under a suitable temperature and pressure supplied from a suitable source (not shown). The ports 42 are located above the fuel bed and directed across the grate surface and above the fire, as shown in dotted lines. This causes a vortex over the fire and insures thorough mixture of the gases of combustion with the air and complete combustion of the gases.

In the operation of my apparatus, fuel such as coal is forced into the retort 3| =by the plunger 32 and is fed along the length of, its upwardly sloping bottom by the reciprocating rams I8, which rams .also by their reciprocation force the green coal up into the coking zone 44' (Figure 5) and break up the semi-coke adhering to the hot side of -the coking zone. Air passes through the holes I3 in tuyres 8 through the layers of ash'45 on the tuyeres and into the fuel 4I. Ash is discharged continuously to the ash pit I from the top surface of the ash pit end of the tuyre stack V5, underneath the layer of fuel, by the reciprocating action of the tuyre stack 5.

Secondary air is admitted through the ports 42 in close proximity to the top of the fuel bed where each other with a quick return motionso that the fuel thereon is forced along the grate from the front toward the rear. Fuel is fed from the hopper 3|) into the retort- 3| whence it is forced by the main ram or plunger 32 into the stoker retort 1.

As explained above, the fuel is moved through and along the stoker retort by the step-like motion of the secondary rams. The main ram 32 is reciprocated back and forth by the link 33, which is gear connected with the shaft 20. The rate of movement of the secondary rams depends von 4the operating conditions of the stoker.

' Combustion air under pressure is admitted to the plenum chamber or wind box 34 from an air "the thorough mixing of the rich gases rising from the bed by the tangential firing of the overflre air burns up the combustible gases and any line dust that is blown through the bed.

Following the teachings of my invention, it is possible to obtain very much higher burning rates than are possible with conventional 4designs of stokers and an additional increase in capacity available within a given space is possible because of the small space required for ash removal.

Boilers equipped with stokers made according to duct 35 to which air is supplied under temperature and pressure from a suitable source (not The flow of air fromthe air duct 35 to the plenum chamber 34 is controlled by slide valves 36, which open and close the ports 31 in the plate which divides the wind box 34 from the air duct 35. The slide valves 38 are reciprocated by the connecting rod 39, which is moved through a crank connection 39a by the shaft 40. The shafts 20 and 40 are connected together by a drive 4|, indicated in chain lines, and are driven by a prime mover (not shown). The slide valves 30 are opened and closed at the same frequency as the tuyre stacks 8 are reciprocated. The mechanism for reciprocating the tuyre stacks I and the mechanism for operating the slide valves are so arranged that the phase of operation of each relative to each other may be adjusted. To

make the parts adjustable, set screws |00 as shown in Figs. 1 and 2 may be provided to adjustably connect the crank 39a to the shaft 40 and to adjustably connectthe cam 2| to the shaft 20. In

adjusted that the rate of flow of the fuel and of my invention will have a capacity considerably greater per square foot of grate area than that obtained with any stoker of similar grate area heretofore made. 'I'he rate of burning per unit of grate area is increased and the cost of installation is reduced, due to the lower cost of grates or tuyres. Maintenance cost of the grates or tuyres is lowered because replacement of grates will be reduced by the improved cooling due to the high rates of air flow required. and because the layer of ash on the grateswill protect them from heat radiated downwardly toward them from the burning fuel. Slagging of the tubes in the boilers by molten ash carried up to the tube by the hot gases rising from the fuel bed will be reduced because ash will not be subjected to high temperatures when it is liberated in the upper part of the bed, because the uniformity and depth of the bedfwill permit the endothermic reaction C+COz-)CO to be completed to a considerable extent. This` will materially' cool the upper parts of the bed to temperatures well below the fusion point of most ash.

It must also be observed that this method of firing lends itself to the use of highly preheated air with stoker firing. It would be necessary where such practice is desired to preheat only the secondary, overre air while the air for combustion supplied through the tuyres would be air at normal atmospheric temperature. Since the secondary overiire air is not passed through the I grates, the increasing maintenance costs found with increasing air temperature where all the air is passed through the grates would not ,limit the preheat temperature that might be used.

air were heated to a high temperature, say above the order of 300 F., this would tend to materially decrease the life of the grates.

While I have set forth a preferred embodiment of my invention. it is to be understood that my invention is not limited to this embodiment, but may be practiced within the scope of the following claims.

I claim:

1. A multiple retort underfeed stoker for burning coal having alternate parallel fuel retorts and tuyre stacks in side by side relationship wherein the width of the retorts does not exceed 50% of the width of the tuyre stacks, each of said fuel retorts having a plurality of secondary rams, the ends of which are in overlapping relationship, means for reciprocating the secondary rams in a step-like motion relatively to each other so that fuel thereon is moved from the front toward the rear of the grate and means for reciprocating the tuyre stacks relatively slowly in a direction towardvthe rear of the stoker and relatively quickly from the rear toward the front.

2. A multiple retort underfeed stoker for burning coal having alternate parallel fuel retorts and tuyre stacks in side by side relationship wherein the width of the retorts does not exceed 50% of the width of the tuyre stacks, said fuel retorts having a plurality of secondary rams, the ends of which are in overlapping relationship, means for moving the secondary rams in a -step-like motion relatively to each other so that fuel thereon is moved from the front to the rear of the grate, means for supplying combustion air through the tuyre stacks. valves for controlling the flow of combustion air, and means for opening and closing the valves in synchronism with the movement of the tuyre stacks.

3. A multiple retort underfeed stokerv for burning coal having alternate parallel fuel retorts and tuyre stacks inside by side relationship wherein the width of the retorts does not exceed 50% of the width of the tuyre stacks, said fuel retorts having a plurality of secondary rams, the ends of which are in overlapping relationship, means for moving the secondary rams in a step-like motion relatively to each other so that fuel thereon is moved from the front to the rear of the grate, means for supplying combustion air through the tuyre stacks, valves for controlling the flow of combustion air, and means 1 for opening and closing the valves in synchroA nism with the movement of the tuyre stacks, i

the valves being open .for at least aportion of the time the tuyre stacks are being moved toward the rear of the grate.

4. An underfeed -stoker for burning coal having alternate fuel retorts and tuyre stacks in sid by side relationship, the width of the retorts and the tuyre stacks having a relationship expressed by the formula foot per hour of actual air-admitting surface determined by theformula:

G =rate of air flow pounds/square foot/hour M=stoichiometric factor 0.414

P1=oxygen partialpressure atm. (in air=0.21)

u1 :rate of reaction C-i-Oa-COz pounds/cubic foot/hour i u2 :rate of reaction CO2+C 2CO pounds/cubic feet/hour f :depth of ignited fuel bed in feet y e =base of natural logarithms,

and wherein the values of G and f are such that they produce a value of U* substantially greater lthan 110.

5. A method of 'burning coal in a multiple retort underfeed stoker which comprises moving the coal along a plane in parallel rows and introducing air through the bed between said rows, agitating the coal on the bed so as to prevent the formation of solid coke walls between the rows of green fuel, and partially suspending the fuel bed by an intermittent stream of air flowing through it 6. 'I'he method of burning `coal in a mechanical stoker, which comprises intermittently passing combustion air through coke beds lying between separatedgreen. coal masses, continuously supplying overre 'combustion air over the fuel bed, and introducing the last mentioned air over the fuel bed in intersecting streams under pressure so as to cause the air to swirl above the fuel bed.

7. In a multiple retort underfeed stoker for burning coal, a stoker area composed of alternate parallel fuel retorts and reciprocable tuyre stacks in side by side relationship, the width of each of the retorts not exceeding 50% of the width of each of the tuyre stacks, e'ach of said retorts having a plurality of secondary rams disposed lengthwise of the retorts, means for moving the secondary rams in a step-like motion relatively to each other so that'fuel 'thereon is moved from the front-to the rear of the stoker area, means for supplying combustion air through the tuyre stacks, means for reciprocating the tuyre stacks, valves for controlling the ow of combustionl air, and means for opening and closing the valves in synchronism with the movement of the tuyre stacks. v

8. In a multiple retort underfeed stoker for burning coal, a stoker area composed of alternate parallel fuel retorts and reciprocable tuyre stacks in side by side relationship, the width of each of the rotorts not exceeding 50% of the width of each of the tuyre stacks, each of said retorts having a plurality of secondary rams disposed lengthwise of the retorts, means for movl ing the secondary rams in a step-like motion relatively to each other so that fuel thereon is moved from the front to the rear of the stoker area, means for supplying combustion air through the tuyre stacks, ymeans for reciprocating the tuyre stacks, valves for controlling the flow of combustion air, and means for opening and closing the valves in synchronism with the movemerit of the tuyre stacks, the valves being open for at least a portion of the time the tuyre stacks are being moved grate.

toward the rear of the' 9. In a multiple retort underfeed stoker for burning coal, a stoker area composed of alternate parallel fuel retorts and reciprocable tuyre stacks in side by side relationship, the width of each of the retorts not exceeding 50% of the width of each of the tuyre stacks, each of said.l

retorts having a plurality of secondary rams disposed'lengtliwise of the retorts, means for moving the secondary rams in a step-like .motion where d is the width of the air-admitting surfaces in feet, w is the width of the green coal masses in feet, B is a constant whose value lies between 65 and 120 per foot depth of fuel bed and U* is the theoretical combustion rate of coal per pound per square foot per hour of actual air-admitting surface determined by the for- G :rate of air flow, 1in/sq, fit/hr. M=stoichiometric factor, 0.414

. Pi=oxygen partial pressure atm. (in air=.0.21)

burning coal, a stoker area composed of alternate parallel fuel retorts and reciprocable tuyre stacks in side by side relationship, the width of each of the retorts not exceeding 50% of the width of each of the tuyre stacks, each of said retorts having a plurality of secondary rams disposed lengthwise of the retorts, means for moving the rsecondary rams in a step-like motion relatively to each other so that fuel thereon is moved from the front to the rear of the stoker area, means for supplying combustion air through the tuyre stacks, means for reciprocating the tuyre stacks, valves for controlling the flow of combustion air, means for opening and closing the valves in synchronism with the movement of the tuyre stacks, and meansfor supplying from to 70% of the air for combustion tangentially over the fire and under pressure.

` tuyre stacks, and means for intermittently supplying combustion air through the tuyre stacks.

12. In a multiple retort underfeed stoker for burning coal, a stoker area composed of alternate parallel'fuel retorts and tuyre stacks in side by side relationship, the width of each of the retorts not exceeding 50% of the width of the tuyre stacks, said retorts extending substantially the full length of the stoker area, means closing the ends of the fuel retorts, means in each of said retorts for urging the fuel therein upwardly and toward the rear of the retort, means for imparting reciprocatory motion to the tuyre stacks, means for intermittently supplying combustion air through the tuyre stacks, and means for supplying from 25% to 70% of the air for combustion over the fire.

13. In a furnace, in combnati0n, a multiple retort underfeed stoker for burning coal, comprising a stoker area composed of alternate parallel fuel retorts and reciprocable tuyre stacks in side by side relationship wherein the Width of the retorts and the tuyre stacks have a relationship expressed by the formula u1=rate of reaction C+Oz CO2, lbs/cu. ft./hr. u2=rate of reaction CO2+C 2CO, lbs/cu. ft./hr. f :depth of ignited fuel bed in feet e :base of natural logarithms and wherein the values of G and f are such that they produce a value of U* substantially greater than 110, said fuel retorts having means extending lengthwise of the retorts for moving the fuel upwardly and toward the rear of the retorts, means for supplying a portion of the combustion air lthrough the tuyre stacks, and means for reciprocating the tuyre stacks.

14. In a furnace, in combination, a multiple retort underfeed stoker for burning coal, comprising a stoker area composed of alternate parallel fuel retorts and reciprocable tuyre stacks in side by side relationship, wherein the width of the retorts and the tuyre stacks have a relationship expressed by the formula where d is the width of the air-admitting sur- G :rate of air iiow, lbs/sq. ft./hr. M=stoichiometric factor, 0.414 P1=oxygen partial pressure atm. (in air=0.21)

'u1=rate of reaction C+O2- CO2, lbs./cu. ft.'/hr.

u2= rate of reaction COa+C- 2CO, lbs./cu.- ft./hr. f :depth of ignited fuel bed in feet :base of natural logarithms and wherein the values of G and fare such that they produce a value of U* substantially greater than 110, said fuel retorts having means extending lengthwise of the retorts for moving the fuel 15. An underfeed stoker for burning coal having alternate fuel retorts and tuyre stacks in side by side relationship, the width of the retorts and the tuyre stacks having a relationship expressed by the formula where d is the width of the tuyre stack in feet.

yw is the width of the retort in feet, B is a constant whose .value lies between 65 and 120 per foot depth of fuel bed, and U* .is determined by the formula and the quantities d and w are to be so chosen that U has a value substantially in excess of 75.

16. An underfeed Stoker for burning coal having alternate fuel retorts and tuyre stacks in side by side relationship. the widthv of the retorts and the tuyre stacks having a relationship` expressed by the formula B dw=w l where d is the width of the tuyre stackin reet, w is the width of the retort in feet. B is a constant whose value lies between 65 and 120 per foot depth of fuel bed. and U is determined by the formula d U- U* d w and the quantities d and w are lto be so chosen that U has a value substantially in excess of 75 but not greater than the value of U determined by the formula:

where G =rate of air ow, lbs/sq. ft./hr. M==stoichiometric factor, 0.414l

P1=oxygen partial pressure atm. (in air=0.21) u1=rate of reaction C+.O2- C0n, lbs/cu. ft./hr. uz=rate of reaction CO2+C 2CO, lbs/cu. ft./ hr. l =depth of ignited fuel bed in feet e y =base of natural logarithms 17. An underfeed stoker for burning coal havfoot depth of fuel bed, and U* is determined by the formula d U Wm and thequantities d and-w are to be so chosen that U has a value substantially in excess of 75 but notgreater than the value of U* determined A by the formula:

where y G =ratefof air ow. lbs/sq. ft./hr. M=stolchiornetric factor, 0.414

Pi=oxygen partial pressure atm. (in air=0.21) ur=rate of reaction C+O2 C02, lbs-Jou. ft./hr.'

f =depth of ignited fuel bedyin feet e =base of natural logarithms and in which G has the value at which the coke bed is in the just-moving condition. 

